Many scientists are searching for extraterrestrial life on planets with conditions similar to Earth. For example, a habitable planet would likely have water. For liquid water, the planet would need to be at the perfect distance from its sun to prevent the water from freezing and turning into gas.
There is currently no evidence of life on other planets, but as scientists discover more planets outside our solar system, they are hopeful that some of these worlds will prove “perfectly suited” for life to exist or evolve.
Has extraterrestrial life been discovered?: Not yet!
Where are scientists looking for aliens?: Water-rich bodies in our solar system, such as Jupiter’s moon Europa, and Earth-like exoplanets—planets outside our solar system.
How many planets in the Milky Way have suitable conditions for life? Estimates put the number at around 300 million.
Scientists have been listening for alien signals using special radio receivers since 1992. So far, they haven’t detected anything!
Life may have once existed on Mars—most likely in the form of tiny creatures like bacteria—but scientists can’t say for sure.
Jupiter’s moon Europa has an ocean and hydrothermal vents, or cracks in the seafloor through which hot water seeps. Scientists believe that life on Earth may have originated in hydrothermal vents.
The “Goldilocks zone” is the region around a star where temperatures allow liquid water to exist. Many scientists believe that planets in the Goldilocks zone are most likely to support life.
The oldest known life on Earth is 4.2 billion years old.
Scientists take the possibility of extraterrestrial life seriously. The Milky Way galaxy contains approximately 100 billion stars, and the universe contains at least 2 trillion galaxies available for exploration. If most of these stars are orbited by at least one planet, then the cosmos could contain up to 20 billion trillion extraterrestrial worlds.
NASA’s Kepler space telescope, just before entering orbit, following Earth around the Sun. The telescope is one of the key tools astronomers use to discover exoplanets, or planets outside our solar system. NASA/KSC
Given these numbers, it would be shocking if life existed only on one planet—Earth. But our closest neighbors in the solar system, Mars and Venus, appear to be lifeless. Some moons of Saturn and Jupiter have water, so life may exist there—most likely tiny creatures the size of microbes. If Earthlings ever encounter aliens face to face, they’ll likely need a microscope to say hello.
Until scientists find compelling evidence, such as a communication signal from an alien planet or fossilized microbes from Mars, Earth will remain the only planet known to harbor life.
The appearance of aliens would depend on their origin. For example, on the icy moons of our solar system (Jupiter’s Ganymede and Europa, and Saturn’s Enceladus), life could thrive around hydrothermal vents in the subglacial oceans. This life could look similar to the bizarre deep-ocean creatures we see on Earth. Primitive microbes, such as the single-celled archaea of Earth, could exist. Relatively simple, multi-celled creatures, like Earth’s tube worms, could also exist, subsisting on chemicals from the fluids released by the vents.
Earth formed about 4.5 billion years ago, and we think the first life existed about 4.2 billion years ago. But life on Earth began simply and remained that way for a long time. The first carbon-producing microbes appeared at least 3.7 billion years ago. Carbon is an element that is part of all known life. But the type of cells that gave rise to animals, plants, and other complex life forms only appeared between 2.7 billion and 1.8 billion years ago. Life forms consisting of many cells appeared only 600 million years ago. And modern humans only appeared on the scene about 300,000 years ago.
This means that if other planets harboring life are similar to Earth, the timeframe for intelligent life (or even something as cute as a koala) to exist on them is quite short. However, there is a high probability that human life could intersect with microbial life on another planet.
Scientists believe that life on other planets will evolve under the same influence as on Earth—namely, evolution. Environmental changes force living things to change, leading to the emergence of new, more complex species. Therefore, a planet in space similar to Earth, which has undergone numerous changes in its surface, rocks, and climate, would likely also harbor complex life. In this case, aliens might face the same problems and needs as those here on Earth and, consequently, might develop similar traits. For example, eyes have evolved independently dozens of times on Earth and could evolve during life on other planets.
Some scientists still hope that life exists elsewhere in our solar system. If so, it’s likely on one of these moons:
Ganymede: Jupiter’s largest moon is larger than Mercury and hides a massive ocean beneath its icy surface.
Europa: Another Jupiter moon with an ice-covered ocean, Europa contains liquid water, heat generated by Jupiter’s gravity, and chemicals that are the building blocks of life.
Enceladus: This moon of Saturn spews water vapor containing carbon compounds from its surface. One of these compounds, hydrogen cyanide, plays a crucial role in the origin of life.
Titan: This moon of Saturn is very cold, but its surface contains carbon-rich liquid. Any life on Titan would have to exist in conditions not found on Earth.
Triton: Neptune’s moon, Triton, is extremely cold, but may harbor an ocean beneath its surface ice. It also exhibits geological activity in the form of geysers that erupt when the sun heats nitrogen ice on the planet’s surface.
And our closest neighbor, Mars, may have once supported life, as it had liquid water and an atmosphere. Today, any life would have to exist in deep reservoirs beneath the Red Planet’s surface.
Ultra-powerful telescopes are allowing researchers to discover planets beyond our solar system that could support life. This image shows some exoplanets that could be similar to Earth (from left to right): Kepler-22b, Kepler-69c, Kepler-452b, Kepler-62f, and Kepler-186f. Earth is on the right. NASA/Ames/JPL-Caltech
Scientists are constantly discovering new exoplanets outside our solar system. They can learn more about the atmospheres of these planets by studying the types of light waves they observe with powerful telescopes. One exoplanet that holds promise for life is called K2-18b. This world is too distant for humans, but light from the planet has reached Earth. This light tells us that the planet has an ocean. Scientists believe they have detected chemicals in K2-18b’s atmosphere that could be produced by marine organisms, but they are not certain.
Scientists search for aliens in several ways.
First, they listen for alien signals. This is called “passive SETI” (the search for extraterrestrial intelligence). If aliens are as smart as we are, their technology could send signals into space. For example, on Earth, all the radio waves from our phones, satellites, and TV stations “leak” into space, and these leaked radio waves could be picked up if someone is listening. Therefore, earthlings use telescopes designed to receive radio waves from space in the hopes of detecting extraterrestrial signals.
However, this only works for tech-savvy aliens. Scientists also use light to study molecules present on distant planets and moons. On Earth, some molecules are usually or always produced by living organisms, so if these molecules are detected elsewhere, they could be a sign of life. Such studies allow scientists to search for signs of life on exoplanets too distant for spacecraft to detect.
Scientists are also sending spacecraft to nearby sites where life may exist. For example, rovers are collecting rock samples that may contain traces of fossilized ancient Martian microbes. NASA plans to send a propeller-powered drone called Dragonfly to Saturn’s moon Titan in 2028. Dragonfly is expected to reach Titan by 2034 and begin searching for chemicals associated with life. The European Space Agency plans to send a mission to Enceladus, also to search for signs of past or present life.
Unidentified flying objects (UFOs) are unexplained phenomena in the sky. The first modern UFO sighting dates back to 1947, when an American fighter pilot reported flying saucers in Washington, D.C. Not every UFO sighting can be explained, but many turn out to be events of terrestrial origin. For example, the famous “UFO crash” in Roswell, New Mexico, in 1947 was actually the debris of an experimental military balloon designed to detect sound waves from atomic bomb tests in the Soviet Union.
Recently, strange videos have captured seemingly fast-moving, hovering objects. These “unidentified aerial phenomena” (UAPs) have no official explanation. However, they may be ordinary objects appearing to move quickly due to optical illusions, or objects that are not what they appear. The pilot filming these videos may have seen drones, weather balloons, or even birds.
Any alien civilization with the technology necessary to build spaceships would have to be at a tremendous distance, given that the nearest exoplanet with habitable conditions is Proxima Centauri B, located 24 trillion miles away. Proxima Centauri B is not very close and may not have an atmosphere. Therefore, life may not exist on it at all, let alone life capable of reaching us. And we would need a truly advanced means of getting there: with current Earth technology, a spacecraft would need 6,300 years to travel from Earth to Proxima Centauri B.
A group of scientists has proposed a completely new equation designed to determine the probability of intelligent life emerging in our Universe. The equation is built around two key concepts: star formation and dark energy.
It turns out we may have surpassed all expectations—our Universe may not have been the most likely place for life to emerge.
How many intelligent civilizations exist in the Universe? This is the question the famous Drake equation was designed to answer. Developed by astrophysicist Frank Drake in 1961, the Drake equation uses parameters such as the rate of star formation, the number of planets orbiting a star, and the length of time a civilization could transmit information into space to attempt to derive a probabilistic estimate of how far we are from being alone in the cosmos.
A study published in the journal Monthly Notices of the Royal Astronomical Society in 2024 seeks to take this idea even further, using a probability equation that is essentially the Drake Equation 2.0. But it works somewhat differently: it uses the driving conditions governing our universe to extrapolate the probability of intelligent life, rather than simply considering the results those conditions have already confirmed.
And that’s not all. This team took their equation to a hypothetical multiverse to try to determine what universal conditions would be optimal for intelligent life. Spoiler alert: it’s not our multiverse.
MARK GARLICK/SCIENCE PHOTOGRAPHY LIBRARY/Getty Images
According to the paper, given the ideal balance of dark energy and everything else necessary for star formation, about 27% of the baryonic matter in the Universe will eventually turn into stars. This represents the best chance for life—a Universe in which conditions are suitable for star formation from 27% of the matter.
But, as the model showed, that’s not what we have. In our Universe, only about 23% of the matter turns into stars, mainly because we have slightly less dark energy than seems optimal for star formation. So, in a sense, we’ve exceeded all expectations. The conditions we find ourselves in are not ideal for the origin of life.
“Understanding dark energy and its influence on our Universe is one of the most important tasks in cosmology and fundamental physics,” said lead author Daniele Sorini in a press release. “The parameters governing our Universe, including the density of dark energy, could explain our existence. However, to our surprise, we found that even a significantly higher density of dark energy is still compatible with life, suggesting that we may not live in the most probable universe.”
Nevertheless, we are close to the “most likely” scenario. Star formation increased along with dark matter, until a certain point, when dark energy became too abundant, and star formation plummeted. Thus, in a vast number of multiverses—if multiverses really exist—life would have been much more difficult to originate than in our own universe. Of course, in some universes the odds would be higher, but they were few and far between.
Of course, this doesn’t fully open up the world of the multiverse or guarantee anything about extraterrestrial life. But it does provide an intriguing framework with which scientists can better understand the colossal forces that underlie our universe. “It will be exciting to use this model to study the emergence of life in different universes,” said Lucas Lombriser, co-author of the study, in a press release, “and to see whether we need to rethink some of the fundamental questions we ask about our own universe.”
A 2023 study suggests that life on other worlds may differ from what we know. A team of researchers studied a type of reaction known as autocatalysis (a self-sustaining reaction), which has long been considered a potential enabler for the emergence of life where it previously did not exist.
Previously thought to be quite rare, these reactions are actually quite common, and researchers suggest they may be worth studying in the search for extraterrestrial life.
At the most basic level, we’re searching for life elsewhere in the universe—we’re searching for ourselves. By “ourselves,” we don’t just mean humans. Rather, it refers to all forms of life on Earth. When we look at the stars and the many planets orbiting them, searching for signs of life, we’re not searching for any life at all, but for life as we know it.
In a sense, we absolutely need this. Searching for signs of life is difficult enough, but detecting chemical signatures from other worlds is no easy task. So we’re looking for the signatures of the only life we know for sure exists—carbon-based life, also known as organic life.
But the fact is, we have no guarantee that the Earthly version of life is the only possible one. And a recent study has once again highlighted that we may be limiting our chances of detecting alien life by focusing on the most familiar features.
MARK GARLICK/SCIENCE PHOTOGRAPHY LIBRARY/Getty Images
However, even the team working on this problem started with life as it is, before expanding it. The research focused on what’s known as autocatalysis—a chemical reaction that can sustain itself. Essentially, molecules participating in a reaction produce new molecules that are also capable of sustaining the reaction, which in turn produce even more molecules, and so on. The reaction continues on its own.
This is the key to life on Earth. In fact, it could be said that it is what sustains all life on Earth. “One of the main reasons origin-of-life researchers are interested in autocatalysis is that reproduction—a key feature of life—is an example of autocatalysis,” said Betül Kaçar, one of the study’s authors, in an article on Space.com. “Life catalyzes the formation of new living things. One cell produces two cells, which can become four, and so on. As the number of cells increases, the number and variety of possible interactions correspondingly increases.”
However, this phenomenon is not unique to life. Autocatalysis does not necessarily involve organic compounds. And it has long been believed that autocatalytic reactions can play a significant role in the emergence of life where it previously did not exist.
The problem, however, is that few examples of inorganic autocatalysis have been available to us in the past. And the rarity of these reactions was long considered a sign of their improbability, meaning the likelihood that they could have given rise to life on another world was extremely low.
But the team decided to challenge this idea. So they turned to the depths of science. Using modern search and translation tools, the team was able to sift through two centuries of scientific records and conduct a fairly comprehensive study of documented autocatalytic reactions.
In the end, they discovered 270 such self-sustaining cycles. Not only were some of them inorganic, but most weren’t carbon-based. Some were based on rare or radioactive elements. Some occurred only at extreme temperatures or pressures. And four of them even involved noble gases, which are known to react with virtually nothing.
Going forward, the group hopes that their new library of autocatalytic reactions will be used in experiments that will directly probe the kinds of effects these cycles can have on chemistry—especially given that they can be combined to produce increasingly complex reactions and products.
Extraterrestrial life may be unlike anything we’ve ever seen on Earth. Finding evidence of these unknown organisms will require some ingenuity.
We have only one example of the formation of biology in the Universe—life on Earth. But what if life can form in other ways? How can we search for extraterrestrial life if we don’t know what it might look like?
These questions concern astrobiologists—scientists who search for life beyond Earth. Astrobiologists are trying to formulate universal rules governing the emergence of complex physical and biological systems both on Earth and beyond, reports The Conversation.
Since the first exoplanet discovery in 1995, more than 5,000 exoplanets, or planets orbiting other stars, have been discovered.
Many of these exoplanets, like Earth, are small and rocky, and reside in the habitable zones of their stars. The habitable zone is the range of distances between a planet’s surface and the star it orbits that would allow the planet to have liquid water and, therefore, support life as we know it on Earth.
The sample of exoplanets discovered so far predicts 300 million potential biological experiments in our galaxy—or 300 million locations, including exoplanets and other bodies such as moons, with suitable conditions for the emergence of biology.
The uncertainty for researchers begins with defining life. It seems easy to define life, since we know life when we see it, whether it’s a bird in flight or a microbe moving in a drop of water. But scientists disagree on the definition, and some believe a comprehensive definition may be impossible.
Minerals are an example of a non-living system whose diversity and complexity have increased over billions of years. Doug Bowman, CC BY
NASA defines life as “a self-sustaining chemical reaction capable of Darwinian evolution.” This means that organisms with complex chemical systems evolve by adapting to their environment. Darwinian evolution posits that an organism’s survival depends on its adaptation to its environment.
Exoplanets are distant and hundreds of millions of times fainter than their host stars, making their study challenging. Astronomers can study the atmospheres and surfaces of Earth-like exoplanets using spectroscopy to detect chemical signatures of life.
Spectroscopy can detect traces of oxygen in the planet’s atmosphere that microbes called blue-green algae created through photosynthesis on Earth billions of years ago, or traces of chlorophyll that indicate plant life.
NASA’s definition of life raises a number of important yet unanswered questions. Is Darwin’s theory of evolution universal? What chemical reactions could lead to the emergence of biology beyond Earth?
The same chemical processes are observed in all living organisms on Earth, and these processes may be universal. However, in other places, they may differ dramatically.
In October 2024, a group of scientists from various fields gathered to take a fresh look at evolution. They wanted to take a step back and explore what processes—biological or otherwise—create order in the Universe, in order to understand how to study the emergence of life completely unlike that on Earth.
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Two researchers present at the conference argued that complex systems of chemicals or minerals, when exposed to environments where some configurations are better preserved than others, evolve to store greater amounts of information. Over time, the system will become increasingly diverse and complex, acquiring the functions necessary for survival through a kind of natural selection. They proposed that a law may exist that describes the evolution of a wide range of physical systems. Biological evolution through natural selection is just one example of this broader law.
In biology, information is the instructions stored in the sequence of nucleotides in the DNA molecule, which together make up an organism’s genome and determine how that organism looks and functions. If complexity is defined from an information-theoretic perspective, natural selection would lead the genome to become more complex because it stores more information about the environment.
Complexity can be useful for measuring the boundary between life and non-life.
However, the conclusion that animals are more complex than microbes is erroneous. Biological information increases with genome size, but the density of evolutionary information decreases. Evolutionary information density is the proportion of functional genes in the genome, or the proportion of the total genetic material responsible for environmental adaptation.
Organisms that people consider primitive, such as bacteria, have genomes with a high information density and therefore appear better designed than the genomes of plants or animals.
A universal theory of life has yet to be developed. Such a theory would incorporate the concepts of complexity and information storage, but would not be tied to DNA or the specific cell types found in terrestrial biology.
Researchers are exploring alternatives to terrestrial biochemistry. All known living organisms, from bacteria to humans, contain water, which is the solvent necessary for life on Earth. A solvent is a liquid medium that facilitates chemical reactions that could lead to life. But life could potentially arise from other solvents as well.
Astrobiologists William Baines and Sara Seager have examined thousands of molecules that could be associated with life. Sulfuric acid, ammonia, liquid carbon dioxide, and even liquid sulfur are being considered as potential solvents.
Alien life may not be based on carbon, which forms the basis of all life’s essential molecules, at least here on Earth. It may not even require a planet to exist.
Advanced life forms on alien planets may be so strange as to be unrecognizable. Astrobiologists will have to get creative in their search for life beyond Earth.
One strategy involves measuring mineral signatures on the rocky surfaces of exoplanets, as mineral diversity reflects Earth’s biological evolution. As life evolved on Earth, it used and created minerals for exoskeletons and habitats. The number of minerals present at the origin of life has grown from hundreds to approximately 5,000 today.
For example, zircons are simple silicate crystals that predate the origin of life. Zircon, found in Australia, is the oldest known fragment of the Earth’s crust. But other minerals, such as apatite, a complex calcium phosphate mineral, are biological in origin. Apatite is the main component of bones, teeth, and fish scales.
Another way to search for life other than Earth is to detect evidence of civilization, such as artificial lighting or the industrial pollutant nitrogen dioxide. These are examples of traces of intelligent life, known as technosignatures.
It’s still unclear how and when the first detection of life beyond Earth will occur. It could be made within the solar system, by analyzing the atmospheres of exoplanets, or by detecting artificial radio signals from a distant civilization.
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